Switching circuits typically employ hysteresis to provide noise immunity or a safety margin. U.S. Pat. No. 3,725,673 discloses such a hysteresis circuit. FIG. 1 illustrates the basic features of this bipolar circuit. It switches between a pair of states with a hysteresis created by suitably switching a current I.sub.H.
The circuit of FIG. 1 employs differential portions 11 and 12. Portion 11 amplifies an input voltage V.sub.I, which is here the amount by which a signal voltage V.sub.IN exceeds a reference voltage V.sub.REF, to produce circuit currents I.sub.1A and I.sub.1B whose difference is representative of V.sub.I. The internal configuration (not shown) of portion 11 centers around a pair of Darlington circuits. Each Darlington contains an input PNP transistor whose emitter drives the base of a trailing PNP transistor. The bases of the input transistors differentially receive voltage V.sub.I. Currrents I.sub.1A and I.sub.1B are respectively supplied from the collectors of the trailing transistors whose emitters are connected to a current source to receive a supply current. Portion 12, which is configured the same as portion 11, amplifies a control voltage V.sub.C to produce circuit currents I.sub.2A and I.sub.2B whose difference is representative of V.sub.C.
Currents I.sub.1A and I.sub.1B are supplied to the inputs of a differential-to-single-ended converter 13. Another such converter 14 generates current I.sub.H by subtracting I.sub.2A from I.sub.2B. A switch 15 provides current I.sub.H to ground or to one of the inputs of converter 13. A voltage V.sub.X is supplied from converter 13 at a value that varies largely in proportion to I.sub.1B -I.sub.1A -mI.sub.H where m is 1, 0, or -1 depending on the switching of current I.sub.H. An amplifier 16 amplifies voltage V.sub.X to generate an output voltage V.sub.O which controls the position of switch 15.
Circuit transitions occur when I.sub.1B -I.sub.1A equals mI.sub.H. During a transition, V.sub.I passes through a hysteresis threshold approximately equal to mK.sub.G V.sub.C, where K.sub.G is the ratio of the transconductance of portion 12 to that of portion 11. Only two of the three values of m are normally used in any particular embodiment of the circuit. This gives three different cases: (1) m=0, 1; (2) m=-1, 0; and (3) m=-1, 1. FIGS. 2a, 2b, and 2c respectively show the hysteresis characteristics for the three cases. The vertical axes, for example, represent V.sub.O. The low and high values of m for each case respectively fix its low and high hysteresis thresholds. Their difference is the magnitude of the hysteresis.
In particular, assume that switch 15 is positioned at the high m value to set the circuit at the high threshold. Starting with V.sub.I low, the circuit changes state as V.sub.I rises above the high threshold. The new V.sub.O value causes switch 15 to switch to the lower m value. This sets the circuit at the low threshold. When V.sub.I later drops, nothing happens until it passes the low threshold. At that point, the circuit goes back to its original state. Switch 15 thereby returns to the high m value to reset the circuit at the high threshold.
The circuit of U.S. Pat. No. 3,725,673 has several desirable features. The amount of hysteresis can be controlled by simply adjusting K.sub.G or V.sub.C. Since K.sub.G is a ratio, it is largely independent of temperature and fabrication conditions. V.sub.C can be made largely temperature and process independent. Consequently, the hysteresis thresholds are stable with temperature and processing parameters. The flexibility is, however, somewhat limited because the thresholds are not independently controllable. One of them is either fixed at zero (FIGS. 2a and 2b) or is the negative of the other (FIG. 2c). It would be desirable to overcome this flexibility limitation without sacrificing the preceding advantages.